JPH01122440A - Method for multivalent gradation processing of color image - Google Patents

Abstract

PURPOSE:To achieve cost reduction by reducing the number of multivalent levels, that is, the number of memory bits without losing both of the image qualities of the character and pattern parts of a color manuscript, by differentiating a multivalent number of a specific color from other color. CONSTITUTION:A manuscript to be duplicated is separated into R, G and B by a color scanner 1 and the sensitivity irregularity of an image sensing element or the illumination irregularity of a light source is corrected by a shading correction circuit 2 and the deterioration of the MTF characteristic in a high frequency region is corrected by an MTF correction circuit 3. Reflectivity linearity, density linearity or the like are corrected or converted by a gamma correction circuit 4 and, in a color correction UCR processing circuit 5, the equally divided amounts of yellow, magenta and cyan toners are replaced with black toner by a color correction processing part calculating yellow, magenta and cyan amounts and an UCR processing part 5 for replacing the overlapped part of three yellow, magneta and cyan colors and Y, M, C and Bk data subjected to color correction and UCR processing are subjected to binary or multivalent processing by a gradation processing circuit 6 to be sent to a printer 7.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a method for multi-value gradation processing of color images, and in particular, to a method for forming a half-tone image using multi-value area gradation processing using a multi-value threshold. The present invention relates to a multivalue gradation processing method for color images.

(Prior art) In recent years, in various digital color image forming processes such as color copying machines, color printers, and color printing, multi-value area gradation processing using multi-value threshold values is used to create intermediate gradations using multiple colors. Image forming techniques are often employed. In such a multi-value gradation processing method for a color image, the greater the number of multi-value levels, the more the gradation is improved and the color image quality is improved.

However, if the number of multivalue levels is increased in order to improve gradation, the processing means will rapidly become complicated, and the number of processing circuits will increase, leading to an increase in cost. For example, when performing 3-bit signal processing on 8-level colors of yellow, magenta, cyan, and black, and
4 value levels: yellow, magenta, cyan, and black
If 2-bit signal processing is performed for color, the cost is 3:2. On the other hand, reducing the number of gradations is convenient in terms of cost, but the image quality suffers accordingly, and in order to maintain a certain level of image quality, the number of gradations cannot be reduced unnecessarily.

(Purpose) Therefore, the present invention aims to simplify the circuit by reducing the number of multi-value levels, that is, the number of memory bits, without impairing image quality, thereby reducing costs and speeding up processing. The purpose of the present invention is to provide a multi-value gradation processing method for color images.

(Structure) In order to achieve the above object, the present invention is directed to a color image forming system in a digital image forming system that performs multi-value area gradation processing using a multi-value threshold value and forms a medium rims image using a plurality of colors. The multi-value gradation processing method has a configuration in which the multi-value number of a specific color is different from that of a different color.

In the multi-value gradation processing method for color images having such a configuration, specific colors that do not affect image quality are processed using a reduced number of multi-value levels, and specific colors that do not affect image quality are processed using a reduced number of multi-value levels. Colors are now processed through an unreduced number of multi-value levels.

Examples of the present invention will be described in detail below.

First, if we consider what the number of gradations is, it means the change in brightness of each color. In other words, there are three indicators of color: brightness, hue, and saturation.
Among them, (1) hue is determined by a combination of the adhesion amounts of each color, for example, yellow, magenta, and cyan. The amount of each color deposited is determined by the numerical ratio of color separation signals from a scanner or the like, which is determined by the matrix size of dither processing and the number of multi-value levels corresponding to one pixel of the matrix circle. For example, 4 in a 4X4 matrix
At billion level, 4X4X4 (level) x 3 (color) - 1
92 hues are obtained, and in the 8-value level of the 4×4 matrix, 4×4×8 (level)×3(color)−384 hues are obtained. As for hue, even 192 hues at four-value level have sufficient reproducibility. Moreover, in the case of area gradation, the brightness is changed at the same time as the hue depending on the ratio to the white background.

■Saturation indicates the ratio of the dominant wavelength of each color to white. For example, in an image created with each color of yellow, magenta, and cyan, the spectral reflectance of each color is wide, so it is necessary to control the saturation itself. is difficult and at the same time has no practical meaning. Further, the apparent saturation can be adjusted by adjusting the brightness, that is, the ratio of the amount of each color deposited.

From the consideration of ■ and ■ above,! It is understood that tonality has a great meaning as the number of brightness levels.

Further, (2) Brightness refers to a change in brightness, and since the recording paper is a white background, this changes depending on the coverage of each color toner. However, each color toner has its own brightness, and even at 100% coverage, the brightness does not change significantly.

Therefore, if the amount of adhered black toner, for example, black toner, is controlled, the dynamic range of brightness changes can be increased, and four-color reproduction becomes useful here.

As described above, gradation can be approximately represented by a change in brightness, and this depends on the number of gradations of the amount of adhesion of black, for example, black toner. In particular, UCR processing replaces equally divided layers of yellow, magenta, and cyan color toners with black toner, which increases the brightness of black. This is done by first removing the brightness information from the original color, and then expressing the hue by the difference in the amount of toner adhering to other colors, such as yellow, magenta, and cyan. This can be said to be a process in which brightness is expressed by the amount of black toner attached.

Therefore, whereas conventionally 3-bit, 8-level multi-value processing was used for the four colors of yellow, magenta, cyan, and black, only black is maintained at 3-bit processing, and yellow, magenta, and cyan are processed using 3-bit processing. Even if 2-bit processing is performed, sufficient gradation can be obtained. The cost in this case is reduced to 1/4.

Next, the processing circuit will be specifically explained based on the drawings.

FIG. 1 is a block diagram showing a copying system using the present invention. A document to be copied is separated into R, G, and B colors and read by a color scanner 1. The shading correction circuit 2 corrects the sensitivity unevenness of the image sensor, the illumination unevenness of the light source, etc. The MTF correction circuit 3 corrects deterioration in MTF characteristics of the input system, particularly in the high frequency range.

Examples of digital filters used for this MTF correction are shown in FIGS. 2(a) to 2(e). The γ correction circuit 4 corrects or converts the input data so that it has desired characteristics such as linear reflectance and linear density.

In addition, skin removal and the like are also performed at the same time.

Color correction UCR5! ! The logic circuit 5 corrects the difference between the color separation characteristics of the input system and the spectral characteristics of the coloring material of the output system. The color correction UCR processing circuit 5 includes a color correction processing section that calculates the six amounts of color materials, for example, yellow, magenta, and cyan, necessary for faithful color reproduction, and a color correction processing section that calculates the amount of six color materials, such as yellow, magenta, and cyan, that are necessary for faithful color reproduction, and blackens the portion where the three colors of yellow, magenta, and cyan overlap. and an LJCR processing section for replacement.

The color correction process can be realized by executing a matrix calculation as shown in the following equation.

In the above formula, l1llr, W, and page indicate the correction numbers of B, G, and R. Matrix coefficient a1. is determined by the spectral characteristics of the input system and output system (coloring material). Here, the first-order masking equation is taken as an example, but 102 and 100°C
By using a quadratic term or a higher order term such as, it is possible to perform color correction with high layer accuracy. Further, the calculation formula may be changed depending on the hue, or the Neugeba equation may be used. Either way, Y, M,
The value of C can be determined from sl, 1'2 or the values of B, G, and R.

FIG. 3 shows an example of a color correction circuit.

A ROM table is used here. That is,
Y, which is necessary to reproduce the colors represented by R, G, and B,
The amounts of M and C are calculated in advance, and the calculation results are used as RO.
M501, M502, and M503, respectively, and are configured so that color correction processing is performed in a table-reference manner. Furthermore, in color correction calculations, by taking advantage of the fact that complementary colors have the largest contribution, it is possible to reduce the number of bits of other colors compared to the number of bits of complementary color data, thereby reducing ROM space without compromising calculation accuracy. can. In the example shown in Figure 3, the complementary color is set to 7 bits, and the other colors are set to 5 bits x 2, so that 6-bit correction data can be obtained by addressing with a total of 17 bits of data. . As the ROM, a 1 megabit ROM having a configuration of 8 bits per word can be used. Further, it can also be constructed using a plurality of ROMs with a small capacity such as 256 kilobits.

On the other hand, the above UCR processing is performed by calculating the following equation.

Y'-Y-a ・m1n(Y, M, C) lyl'=
lyl-a-min(Y, M, C)(:,'=(:
, -a -m1n(Y, M, ff)3=α-mi
n(Y, M, C) In the above formula, α is a coefficient that determines the amount of UCR, and α
-1 results in 100% UCR processing.

α may be a constant value or may be variable depending on the concentration level. For example, by setting α close to 1 in high-density areas and close to O in highlight areas, the image in the highlight areas can be made smooth.

FIG. 4 shows an example of the LJCR circuit.

Each of the Y, M, and C data after color correction is subtracted by the Bk amount calculated by the Bk amount ratio output circuit 504. Further, FIG. 5 shows an example of a Bk quantity ratio output circuit. Comparators 505, 506 and selector 50
7. Y, M at 508.

The minimum value of C is found, and the ROM is
The quantity is calculated in the table reference formula [3]. Here, the configuration is such that UCR processing is performed after color correction processing, but the Bk amount is calculated from the minimum value of π, G, r3,
It is also possible to perform the color correction process using a value obtained by subtracting 3k from 100.

Returning to Figure 1, Y, M, color corrected and UCR processed,
The C and Bk data are subjected to binary or more multivalue processing in the gradation processing circuit 6. A systematic dither method is generally used as a multi-value processing method, but other methods such as an error diffusion method can also be used. In the present invention,
In order to place particular importance on the gradation properties of Bk, gradation processing is performed using a larger number of multivalues than those of Y, M, and C. Here, 1
Converting 3k to 8 values (3 bits/pel), Y, M,
Suppose that C is subjected to quaternary processing (2 bits/ρel). Depending on the required image quality, convert 9k to 8-value, Y,
It is also possible to adopt a configuration in which M and C are binarized.

FIG. 6 shows an example of a multi-value processing circuit using the organized dither method, in which the processing results are written in the memory address addressed by the dither matrix address and the image data, allowing the table reference method to be used. It is now possible to perform dither processing. This circuit is provided for each color of Y, M, C, and Bk, and can perform high-speed gradation processing by processing each color in parallel.

The gradation-processed data is sent to the printer 7, where a reproduced image is output. In particular, in an output system using a laser printer, the gradation processing data is sent to the printer 7 indirectly via the memory 8 rather than directly. Figure 7 shows
1 is a diagram showing the principle of a printer using one photoreceptor drum.

The Y, M, C, and 8 data processed in parallel by the image processing device are stored in the frame memory 701, and the data is sequentially read out from the memory one color at a time, and is sent to, for example, a laser writing unit of the printer 7. 702. Photosensitive drum 70 according to image data
The toner image formed on the transfer drum 704 is transferred to recording paper wrapped around a transfer drum 704. The recording paper is fixed to the transfer drum 704 until the four-color image is transferred. In this case, the color data initially sent to the printer 7 may be configured to be sent directly to the printer without going through the memory 8.

Conventionally, for example, Y, M, C, and Bk are all gradated to 3 bits/1)el, so if an A4 size document is processed at a sampling density of 16 lines/rwx, approximately This would require 48 megabits/color of frame memory. Therefore, 19 for 4 colors
It becomes 2 megabits. In the present invention, for example, Y,
Since M and C are each gradated to 2 bits/pel, the required capacity is 144 megabits (=48 megabits X%<-o, 25-(192-144)/)
This will result in savings. Furthermore, as mentioned above, in the case where the first color does not go through the frame memory, conventionally 144 bits (-48
In contrast, in the present invention, if Bk, which has a large number of bits, is the first color, 96
Megabits (= 48 megabits less).

FIG. 8 shows a four-drum type printer that is equipped with an independent image forming system for each color so as to enable high-speed output. In this case, the four colors will be outputted in parallel, but each drum 713, 723, 733, 7
There is a shift between each color by an interval of 43. To eliminate this deviation, it is necessary to provide a delay memory 705 for delaying data transfer to each write/write unit 712, 722, 732, 742 by a time corresponding to the drum interval. The required amount of delay memory 705 is equal to the drum interval! (wear), ej (-, P+27+3J')
This is equivalent to . For example, if I is 1100a, sampling density is 16 lines/M, and feed is A4 horizontal feed (main scanning direction width 297 IIR), 137 megabits (-(297as+X 16 lines/#I)
siX6X 16 lines/am)X3 bits). Using the present invention, if the first color is Bk, which has a large number of bits, it is only necessary to delay 61 equivalents with 2-bit data.
91 megabits (= (297amX 16 lines/s+)
X (100 shoes x 6 x 16 pieces/am) x 2 bits)
, and save memory.

In the above description, 3 bits are used for Bk and 2 bits are used for Y, M, and C.

Needless to say, further savings can be achieved by setting M and C to 1 bit. Also, if Bk is the first color,
Frame memory and delay memory can be eliminated,
Even if the number of multi-values of Bk is increased by that amount, the cost of the memory will not increase.

The above has described the Bk component versus the Y, M, and C components, but
Next, the characteristics required between the Y, M, and C components will be explained. When expressing gradations using the systematic dither method, increasing the dither matrix increases the number of gradations, but the resolution decreases.

Conversely, if the matrix is made smaller, the gradation will be reduced. Multi-leveling allows the gradation to be improved without increasing the matrix size. However, as the amount of data increases, the capacity of the puffer memory increases. Therefore, in the above embodiment, we considered a method of increasing the number of multi-level components only to 3, which has a large effect on the resolution of characters, etc. be. On the other hand, conversely, it is also conceivable not to reduce the number of multivalues of components that have little effect on resolution.

In recent years, image information has become more and more colored, and text and line images are now generally colored not only in black but also in colors such as red, blue, and green. Therefore, it is necessary to maintain resolution for color components as well. However, among the Y, M, and C components, the Y component has a characteristic of being visually inconspicuous and having low resolution. Therefore, the multivalue number of Y component is C
, M components can also be reduced. In this case, among the colored characters, the green and red characters are C and Y, M and Y, respectively.
It is reproduced by overlapping color plates, but even if the resolution of Y is low, if the characters are reproduced so that the resolution of C and M colors is high, it is possible to recognize green or red characters. .

Here, as the amount of data after gradation processing (number of bits per pixel), from 3k for 3 bits and 2 bits each for Y, M, and C, the number of bits for Y is reduced and Bk is 3 bits. , M, and C are each 2 bits, and Y is 1 bit. Let us calculate the memory capacity. First, for one drum, if 3k is used as the first color without going through the memory, 80 megabits (= (297mIX 16 lines/a
+)X (210amX16 lines/M)X (2+2+1)
bits), which is 17% (=0.17 - 9" for 4 drums) compared to the 96 megabits required when 3k is 3 bits and Y, M, C are 2 bits each. Then, by setting Bk as the first color and Y as the fourth color, the puff memory can be reduced the most.In this case, 68 megabits (-(297 MRX 16 lines/a+) x (100m x 16 lines/aa+) x (
1X2 + 2X2 + 3X 1) bits), which is 25%C-0,
25=(9168)/91) memory can be saved. In addition, to improve the quality of colored characters, B
If k, M, and C are each 3 bits and Y is 1 bit, then 91 megabits (=(297sm x 16 lines/am)
) X (100amX 16 lines/m) X (1X3
+2X3+3X1') bits), and the capacity is the same as when 8k is 3 bits and Y, M, and C are each 2 bits. In this way, by reducing the number of bits (the number of multivalued bits) of the Y component, it is possible to save memory without deteriorating image quality.

When transmitting data externally, such as by facsimile,
When saving (file) to an external storage device, the amount of data of the components must also be taken into account. At this time, ignoring some deterioration in image quality, set Bk to 3 bits, Y, M, and C to 3 bits.
1 bit, 8k as 2 bits, Y, M, C as 1 bit.
By converting data into bits, it is possible to significantly reduce the amount of data. In this way, in a system that performs not only image formation such as copying, but also transmission, storage, etc., multiple modes are provided, and in the case of a copy mode that requires higher image quality, 3-pit Bk, 3-pit Y, M.

In addition to setting C to 2 bits, if transmission or storage efficiency is important, Bk should be 2 bits, Y, M.

It is convenient to configure the system so that gradation processing is performed using C as one bit. In addition, in transmission/storage mode, you can select [
Processing in which 3k is 3 bits and Y, M, and C are 1 bit,
It is also possible to adopt a configuration that allows processing to use 3 bits for Bk and 2 bits for Y, M, and C. In this case, if the processing conditions (1Si tonality, image size, etc.) are attached as a header to the image data, the image can be reproduced without fail even when the received data or stored data is read out.

FIG. 9 shows a block diagram of a reproduction system having copy modes. This system is controlled by a system controller 11 having a CPU. The operator can specify the mode and other processing conditions using switches (not shown) on the operation panel 12.

In the copy mode using the internal scanner, the scanner 13 is activated and the original is separated into R, G, and B colors and read.
The image processing unit 14 performs the above-mentioned shading correction, M
TF correction, γ correction, color correction, LICR processing is performed,
Multivalue processing is performed by the gradation processing circuit shown in FIG.

At this time, according to the mode selection signal from the system controller 11, Bk is 3 bits, Y, M, and C are each 2 bits.
Multivalued into bits. The multiplexer 15 with
This is for selecting data from the internal scanner 13 or data sent through the external I/F 16. In the copy mode, image data based on the data from the internal scanner 13 is selected. The next data converter 17, when data with a different number of pits (multi-level number) is input from the outside,
Bk3 pit, Y, M, to suit the characteristics of the output system.
C is for converting each into 2-bit data. By addressing image data and a mode selection signal using ROM or RAM, conversion processing can be performed in a table-reference manner.

The output data from this converter 17 is sent to a printer 19 via a memory 18 so that a reproduced image can be obtained.

In the reception mode, image data sent from an external transmitter is received according to header information, and a reproduced image is output from the printer 19 through the multiplexer 15, data converter 17, and memory 18. In the reception mode, first, header information is received through the signal line H, and the information is analyzed by the system controller 11. According to the header information, the system controller 11 generates a more detailed mode selection signal. The image data entered after the header is
The signal is input to the data converter 17 through the multiplexer 15. Here, the image data is converted into image data having a predetermined number of pits according to the mode selection signal described above, and is sent to the printer 19. In this mode, the system controller 11 controls the scanner 13 so that it does not operate.

Next, in the transmission mode, the internal scanner 13 is operated in the same manner as in the copy mode, and the original is separated into R, G, and 8 colors and read, and the image processing section 14 performs the above-mentioned shading correction, MTF correction, and γ correction. , color correction, and UCR processing, and multivalue processing is performed by the gradation processing circuit shown in FIG. At this time, the number of pits (multi-value number) according to the processing conditions selected by the operator through the operation panel 12
Gradation processing is performed in . Processing conditions are transmitted as header information from the system controller 11 prior to image data. Thereafter, the image data that has undergone predetermined gradation processing is transmitted. At this time, the printer 19 is controlled not to operate.

The exchange of data with the external storage device is basically the same as in the transmission mode or reception mode, so the explanation will be omitted.

As explained above, according to this embodiment, in gradation processing of a color image, the Y, M, and C components are gradated to a smaller number of bits than Bk, thereby reducing the amount of data while maintaining high image quality. can be reduced. As a result, it is possible not only to save frame memory and delay memory, but also to shorten operation time and save memory when transmitting and storing data. In general, by making the number of multi-values of gradation processing variable, it is possible to further shorten operation time and save memory during transmission and storage.

Furthermore, by increasing the number of Bl pits, not only the gradation but also the resolution of black characters in particular can be improved. Furthermore, since the control circuits for the Y, M, and C write units are simplified compared to the 8K, the cost of the write system can also be reduced.

Although the above embodiment relates to a laser printer,
The present invention can also be applied to other output devices such as thermal head printers using one head or multiple heads. Further, the present invention is not limited to a four-color full-color copying system, but can also be applied to, for example, a red, black two-color copying system, in which black is made into a large number of bits and red is made into a small number of bits and subjected to N11 processing. That is, in a system that reproduces images in multiple colors, by reducing the number of bits for gradation processing of a specific color, the amount of data can be reduced without deteriorating image quality.

Note that the present invention can be applied not only when using a matrix of a fixed size (for example, when using a 4×4 matrix), but also when changing the matrix size to keep the number of reproduced gradations constant. It's Chino. In other words, when using a matrix of a fixed size, the number of tones reproduced will change depending on the number of bits used, but the resolution of the image is determined by the matrix size, so to achieve high resolution, the matrix The smaller the size, the better. However, the human eye's response to horizontal colors is weak, so even if the resolution is reduced, there is little deterioration in image quality. Moreover, as shown in FIG. 7, the horizontal color requires the most delay memory, so by reducing 3 bits to 2 bits for the horizontal color, a very large memory cost reduction effect can be obtained.

(Effects) As described above, the present invention makes the multi-value number of a specific color different from that of loquat, so the multi-value number of a specific color is made different from that of loquat, so that the multi-value number of a specific color can be changed without impairing the image quality of both text and picture parts of a color original. By reducing the number of encoding levels, that is, the number of memory bits, it is possible to significantly reduce costs.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a copying system using the present invention, FIGS. 2(a) to (e) are explanatory diagrams showing examples of digital filters used for MTF correction, and FIG. FIG. 4 is a block diagram showing an example of a UCR circuit. FIG. 5 is a block diagram showing an example of a Bk quantity ratio output circuit. FIG. FIG. 7 is a block diagram showing an example of a processing circuit; FIG. 7 is a diagram showing the basic structure of a printer using one photosensitive drum;
The figure is a diagram showing the basic structure of a four-drum type printer, and FIG. 9 is a block diagram of a copying system having a copying mode. −・：、・、。

Claims (1)

[Claims] 1. Multi-value gradation processing of a color image in a digital image forming system that performs multi-value area gradation processing using a multi-value threshold value to form an intermediate gradation image using a plurality of colors. 1. A multi-value gradation processing method for a color image, characterized in that the multi-value number of a specific color is made different from that of other colors. 2. The multi-value gradation processing method for a color image according to claim 1, wherein the specific color whose multi-value number is different from the multi-colors is set to black. 3. The image forming color is composed of four colors: yellow, magenta, cyan, and black, and the number of multi-values of yellow, magenta, and cyan is set to be smaller than that of black. Range 1st or 2nd
Multi-value gradation processing method for color images as described in Section 1.